Résumé / Abstract

Résumé

Les pucerons pinicoles néarctiques du genre Essigella (Sternorrhyncha, Aphididae, Lachninae) ont été introduits en Europe, Afrique du Nord, Océanie, et Amérique du Sud. Les séquences ADN mitochondriales, nucléaires et endosymbiotiques de 12 populations introduites provenant de trois continents confirment que toutes appartiennent à Essigella californica (Essig, 1909). La variation dans les séquences des introns du gène nucléaire EF-1α a révélé l’existence de quatre groupes distincts. Le Groupe I rassemble une population venant de Chine, où l’espèce est nouvellement rapportée, et plusieurs venant d’Europe (France et Italie) ; le Groupe II est représenté par une population venant d’Argentine ; le Groupe III inclut deux populations provenant du Sud de l’Australie et une provenant de Nouvelle-Zélande ; et le Groupe IV correspond à cinq populations provenant de l’est et du sud-est de l’Australie. Ces résultats indiquent que les populations introduites d’E. californica proviennent d’au moins quatre populations distinctes. Ils montrent aussi que la variation des introns d’EF-1α peut être une méthode pour discriminer les populations de pucerons à reproduction asexuée.

Mots-clés : lignée asexuée, peste sylvicole, espèce envahissante, Lachninae, discrimination de

populations

Abstract

Aphids in the pine-feeding Nearctic genus Essigella (Sternorrhyncha, Aphididae, Lachninae) have been introduced in Europe, North Africa, Oceania, and South America. Mitochondrial, nuclear, and endosymbiont DNA sequences of 12 introduced populations from three continents confirm they all belong to Essigella californica (Essig, 1909). Intron sequence variation of the nuclear gene EF-1α has revealed the existence of four distinct groups. Group I gathers one population from China, where the species is newly reported, and several from Europe (France and Italy); Group II is represented by one population from Argentina; Group III includes two populations from Southern Australia with one from New Zealand; and Group IV corresponds to five populations from Eastern and South-Eastern Australia. These results

indicate that introduced populations of E. californica have at least four source populations. They also show that intron variation of EF-1α can be a method to discriminate populations of asexually reproducing aphids.

Keywords: asexual lineage, silvicultural pest, invasive species, Lachninae, population

discrimination

2.2 Introduction

Essigella Del Guercio, 1909 (Aphididae, Lachninae, Eulachnini) (Chen et al., 2016) is a

Nearctic genus of aphids living on the needles of Pinaceae (Sorensen, 1994). Essigella

californica (Essig, 1909) is the only species introduced outside North America (Sorensen,

1994). It is recorded from France (Turpeau & Remaudière, 1990), Spain (Seco Fernández & Mier Durante, 1992), Australia (Carver & Kent, 2000), New Zealand (Carver & Kent, 2000; Flynn et al., 2003), Brazil (Carvalho & Lazzari, 2000), Madeira (Aguiar & Ilharco, 2001), Italy (Barbagallo et al., 2005), Tunisia (Boukhris-Bouhachem et al., 2007), Argentina (Ortego & Mier Durante, 2012), and Great Britain (Reid et al., 2015). After examination of the source material, the record from Malta (Mifsud et al., 2009) was discarded due to misidentification, the specimen belonging to the genus Eulachnus, not Essigella. Essigella californica has been recorded on over 34 different species of Pinus and on some other Pinaceae (Watson & Appleton, 2007; Kimber et al., 2013). Though the genus is not usually economically important in its native range (Sorensen, 1994), nor is it known to vector plant viruses (Carver & Kent, 2000), E.

californica was recorded as causing yellowing and defoliation in France (Turpeau &

Remaudière, 1990) and in New Zealand (Carver & Kent, 2000). Although it is not considered a significant pest in New Zealand (Watson et al., 2008), in Australia, E. californica has been associated with severe chlorosis and defoliation across much of the commercial P. radiata D. Don plantation estate and is considered a significant silvicultural pest in that territory (May & Carlyle, 2003; May, 2004; Eyles et al., 2011; Stone et al., 2013a, b). Damage by E. californica was estimated to cause losses of up to AU$21 million per annum to the Australian forest industry (May, 2004), which led to a biological control program using Diaeretus essigellae Starý &

Zuparko, 2002 (Hymenoptera, Braconidae) (Kimber et al., 2010) and the development of resistance breeds of P. radiata for commercial deployment (Sasse et al., 2009).

COI is a mitochondrial gene well known for its use as a DNA barcode in animals (Hebert et al., 2003a, b; Hajibabaei et al., 2006). COI is employed in aphid species identification, notably

in pest control and in phylogenetic analyses (Cœur d’acier et al., 2007; Lee et al., 2011). However, COI shows limits in some aphid groups, sometimes not being precise enough in species delimitation (Cœur d’acier et al., 2014; Lee et al., 2011, 2014). Thus, other genes have been investigated to improve species resolution. The gene Gnd of the obligate bacterial endosymbiont Buchnera aphidicola and the mitochondrial gene ATP6 (Chen et al., 2013; Lee

et al., 2014) were successfully tested. Contrary to mitochondrial genes, many nuclear genes are

more stable (Simon et al., 2010). They are often useful in phylogenetic analyses of higher level arthropod taxa (Caterino et al., 2000; Simon et al., 2010). For example, the nuclear gene EF-1α has been employed in the phylogeny of Hexapoda (Djernæs & Damgaard, 2006). Moreover, the variation of the EF-1α exon–intron structures has proven to be efficient in low level phylogenetic studies as well (Simon et al., 2010). It was successfully used in phylogenetic reconstructions in several groups of insects (Cho et al., 1995; Condamine et al., 2013; Lin et

al., 2013; Cooper et al., 2014) and notably in aphids (Moran et al., 1999; Normark, 1999; von

Dohlen et al., 2006; Kim & Lee, 2008).

DNA analyses have revealed the existence of cryptic species of aphids (Depa et al., 2012; Lee et al., 2015). Often, the only biological distinction between morphologically identical species is their preferred host (Heie, 1986; Lee et al., 2015; Mróz et al., 2015). Essigella is a genus with a difficult taxonomy. Species are morphologically similar and several show high intraspecific variation (Sorensen, 1994). For these reasons, Essigella californica can be morphologically confused with E. hoerneri Gillette & Palmer, 1924 and E. pini Wilson, 1919 (Sorensen, 1994; Barbagallo et al., 2005). However, several elements permit distinction between those species in North America. Essigella californica occurs in the same geographic vicinity as E. hoerneri but usually does not colonize pines of subsection Cembroides as does E.

hoerneri (Sorensen, 1994; Blackman & Eastop, 2017). As a result, if the host is well-identified,

there are few risks of misidentification between the two aphid species. In contrast, the respective native geographic ranges of E. californica and E. pini do not overlap (Sorensen, 1994), making their species identification straightforward. However, these two species are known to share

several pine species as hosts (Sorensen, 1994; Barbagallo et al., 2005); thus, host identity is less useful for identification outside North America. The difficulty in confirming the identity of invasive populations of Essigella species has important repercussions for pest management, especially insofar as an authoritative identification is needed when searching for potential biological control agents.

Intraspecific morphological variation based on host plants was documented in E.

californica by Sorensen (1994). This variation was considered either as being purely

intraspecific (Sorensen, 1994), or suggestive that E. californica was actually one of a complex of species (Carvalho & Lazzari, 2000). No molecular systematic study has been carried out on the genus Essigella, and the existence of cryptic species within the putative E. californica complex has not been fully evaluated. Additionally, because of the possible confusion between

E. californica and other Essigella species, and because the identity of the introduced populations

was deduced only by morphology, there is no confirmation that those populations belong to a single species and that this species is indeed E. californica.

In this study, we used four genes, ATP6, COI, EF-1α, and Gnd, from 12 introduced populations of Essigella in order to confirm whether they belong to the same species and if so, that this species is E. californica. For this purpose, our results were compared with sequences from four North American populations of E. californica and from three other species-level taxa,

E. fusca ssp. voegtlini Sorensen, 1994, E. hoerneri, and E. pini.